An electrophotographic technology is widely used in the fields of a copier, various printers, and the like because an image having immediacy and high quality is obtained, for example. Regarding an electrophotographic photoreceptor (simply also referred to as “a photoreceptor” below) as the core of the electrophotographic technology, a photoreceptor which uses an organic photoconductive substance is used. The organic photoconductive substance has an advantage, for example, that forming a film without pollution is easily performed, and manufacturing is easily performed.
In an organic electrophotographic photoreceptor, in a case of a so-called function-separation type photoreceptor in which functions of generation and moving of charges are divided up to compounds which are separate from each other, a range of materials to be selectable is wide and characteristics of the photoreceptor are easily controlled. Thus, the function-separation type photoreceptor becomes the mainstream in development. From a viewpoint of a layer configuration, a single-layer type electrophotographic photoreceptor (referred to as a single-layer type photoreceptor below) and a laminate type electrophotographic photoreceptor (referred to as a laminate type photoreceptor below) are known. In the single-layer type photoreceptor, a charge generating material and a charge transport material are contained in the same layer. In the laminate type photoreceptor, the charge generating material and the charge transport material are respectively contained in layers (charge generation layer and charge transport layer) and the layers are stacked on each other.
In a case of the laminate type photoreceptor, on the design of the photoreceptor, optimization of a function for each layer is easily achieved, and control of characteristics is also easily performed. Thus, most of the current photoreceptor has this type. In many of such a laminate type photoreceptor, a charge generation layer and a charge transport layer are stacked on a conductive support in this order. Regarding the charge transport layer, the number of suitable electron transport materials is very small, but many material having good characteristics are known as a hole transport material. Thus, a negative charging method is employed in a laminate type photoreceptor using such a hole transport material. The hole transport material is improved with high speed and high image quality of the recent printer, copier, and the like, and thus it is realized in the negative charging method, that a residual potential is significantly reduced (PTL 1).
Contrarily, all of the negative charging method and a positive charging method can be used in a single-layer type photoreceptor. If the positive charging method is used, it is possible to suppress an occurrence of ozone which is a problem in the laminate type photoreceptor, to be small. Thus, electrical characteristics in the positive-charging single-layer type photoreceptor are worse than those in the negative-charging laminate type photoreceptor, in many cases. However, some of positive-charging single-layer type photoreceptors are commercially used as a positive-charging single-layer type electrophotographic photoreceptor (PTL 2).
Even in a positive-charging type image forming apparatus, size reduction, high sensitivity, and high durability of the apparatus are examined in accordance with the current request. For example, regarding size reduction, the following technology is known (PTL 3). That is, in a single-layer type electrophotographic photoreceptor in which a memory image is not generated even in an image forming apparatus which does not include an erasing process, a photosensitive layer contains a phthalocyanine compound as a charge generating material, a hole transport agent, and an electron transport material, in a binder resin. The specific amount of the phthalocyanine compound is contained. The film thickness of a photosensitive layer is 10 to 35 μm. A difference of an absolute value in sensitivity between a positive polarity and a negative polarity which are measured under a predetermined condition is set to be equal to or less than 500 V (PTL 3).
Regarding high sensitivity, a technology in which a photosensitive layer is provided is disclosed (PTL 4). In the photosensitive layer, the half decay amount at a time of positive charging is equal to or less than 0.18 μJ/cm2, and the half decay amount at a time of negative charging is twice to 12 times the half decay amount at a time of positive charging. Further, a technology in which a filler is contained in a photosensitive layer is disclosed (PTL 5). The filler is contained in order to reduce an occurrence of friction between a contact charging type charging unit and the surface of a photoreceptor in a case of being used in an image forming apparatus which includes the charging unit. The filler has a volume average particle diameter of 5 nm to 5 μm.